Compensating lens



COMPENSATING LENS Filed Aug. 16, 1960 2 sheets-sheet 1 Aug- 20 1,963 o. scHERzERVE'l-AL 3,101,430

COMPENSATING LENS Filed Aug. 16, 1960 2 Sheets-Sheet 2 11,1111 failli mi? www Maf f MQ United States Patent O 3,101,430 CMPEN SATIN G LENS Otto Scherzer, Darmstadt-Eberstadt, and Karl Heinz Steigerwald, Heidenheim an der Brenz, Wurttemherg, Germany, assignors to Carl Zeiss, Heidenheim an der Brenz, `Wurttexnherg, Germany Filed Aug. 16, 1960, Ser. No. 49,9251

Claims priority, application Germany Aug. 22, 1959 8 Claims. (Cl. 315-31) The present invention relates to a device for compensating for the apertural defect of -a space-oharge-tree electron optical lens of rotational symmetry.

In many devices which operate with a beam of charge carriers, such as Oscilloscopes, television pickup tubes or video amplifiers, and industrial charge-carrier-beam apparatus 4for drilling, milling, heat treating, welding, soldering, surface treating or hardening of material, it is desired to concentrate the beam of charge carriers in the orm of 'a spot which is as small and as high in intensity as possible. In such applications, there is vgenerally used `for the focusing of the beam an electron optical lens which is free of space charge and is of rotational symmetry. However, ideal focusing is prevented bythe lens defects and, particularly, by the apertu-ral defects of third land fth order.

The apertural 'defect fof an electron optical lens, as s known to the art, results trom the fact that the outer lens areas have a greater power of refraction than the central area of the lens. Compensation for the apertural defect of an electrostatic lens by arranging electrostatic correcting elements behind the lens is known to the art. However, such compensation does away with the rotational symmetry iof the lens. As such a correction element corrects the apertural detect of the lens only in a sectional plane, as a rule several correction elements are required. Such elements must be Very carefully constructed and adjusted, making this type of compensation of apertural `defects very expensive. Furthermore, it has the disadvantage that the correction of a beam of charge carriers of large `diameter is possible only with great diiculty.

It is therefore, the primary object of this invention to provide improved :apparatus for the compensation of the -apertural defects of an electron optical lens of rotational sy-mmetry. In accordance with this object there is provided, in accordance with a preferred embodiment of this invention, in for in the vicinity of said lens a correction element `or lens whicheliminates the freedom from charge of the inside of the len-s and has a power of refraction which decreases with an increase in radius.

'Ihe correction element is developed as a netting lens which consists of a nett-ing clamped between two ring electrodes arranged at right angles to the direction of the beam and is insulated from them. Instead of the netting there `can also be provided a thin sheet containing holes of definite size. Similarly it is possible .to have the sheet of diierent thickness at diierent points or to make the diameter of the holes dependent on the distance yfrom the axis. Furthermore, it is often advantageous to arch the netting or the sheet.

[n order to provide better control of the eld produced by the corrective lens it is advisable to arrange on one or both sides of the netting or foil two ring electrodes which lie at diierent potentials and are insulated from eachother.

'Ihe netting lens generates a corrective iield having rotational symmetry. Thus, the compensation of the apertura-1 defect of the electron optical lens can take place in t one stage. Another advantage of the netting lens is that it is possible directly to correct a charge carrier beam of large diameter.

The disturbing action of the net-ting webs of the netting lens can be so -limited by a favorable selection of the dimensions of the netting that the Igain in intensity obtained more than `counteracts this disturbing effect. Ilhe same applies to the lens action of the individual netting meshes and the idefects of the individual lenses. Since there is concerned here the problem of bringing together a beam passing through ments at a single surface element, a limited defect of all elements can be permitted. Als-o, these defects are less serious in applications concerned primarily` with reduction of beam size, in con-trad-istinction Vto such applications as electron microscopy.

The charge induced on the netting and, thus, the lens action of the individual' meshes of the netting may be maintained small if the electrode shapes and the vol-tages are selected so that the field intensities on Iboth sides olf Athe netting differ only little in magnitude and direction from each other.

In order to obtain a compact construction, it is advisable to combine the electron optical .lens andV the netting lens into a single structural unit,V This combination of elements is particularly possible to correction of the yfocusing of a beam of charge carriers of small aperture, since the `correcting action of the netting lens is 'fully effective even if this netting lens is not arranged in the principal plane of the electron optical lens. Therefore in such t pplications it is readily possible ,to place the netting -lens :on the electrode ott the electron optical lens which faces the `beam generating system (esg. the electron gun). Furthermore, it is possible to arrange the netting lens below or within the electron optical lens.

This invention will be more clearly understood :by ref erence to the following description taken in combination with the accompanying drawings, of which: t

FIG.` l is a partially sectioned View of a schematic showing of a charge carrier beam apparatus provided with an electromagnetic focusing lens with associated netting lens in accordance with the present invention;

FIG. 2 is a cross-section through an electrostatic focus ing lens combined `with a nett-ing lens .to form a single structural unit;

FIG. 3 is a section through an electromagnetic focusing lens combined with a netting lens to form a single structural unit;

FIG. 4 is a section through a netting 'lens having a plurality of ring electro des; i

FIG. 5 is a plane view of a netting lens using a sheet perforated with holes of denite size;

FIG. `6 is a section through a netting lens which, 1in-` stead of a netting, contains a sheet `of different thickness at diderent places;

FIG. 7 is a section through a netting lens with arched netting;

FIG. 8 is `a section through another embodiment of a netting lens, and

FIG. 9 -is a plan view of a netting lens using a sheet perforated with holes of varying size.

In FIG. 1, there is shown a charge carrier `beam generating system comprising acathode 1, a modulator electrode 2, and an anode 3. The beam generating system consisting of these three elements produces an `electron beam 4 whichby means of an electromagneticlens 5 is focused on the structural element 6. The structural ele,` ment 6 can be the fluorescent screen of an oscilloscope, the semiconductor coating `of a television pickup tube, the target electrode of a video `ampliiier or, in the.,case of industrial electron beam apparatus, the object which is to be processed.

`'Ihe pole shoes of the `electromagnetic lens 5 are marked 7 and 8. On the upper pole shoe 7, there `is ya large number of surface ele-V placed aA netting lens 9 which consists of a conductive netting or screen 12 clamped between thering electrodes amd Y11 which are arranged at right angles to the direction of the beam. This netting is insulated by the insulator 13 from the ring electrodes 10 and 11.

` In operation, a negative voltage is applied to the netting12'while the ring electrodes 10 and 11 are grounded. As a result of this, there is produced on the netting a v surface charge which has the result that the negative power of refratcion of the netting lens 9 increases in amount with an increase in radius, i.e. that the effective power of refraction decreases as a function of the radius. Since this decrease in the power of refraction takes place with the third power of the radius, it is possible with this netting lens fully to compensate for the apertural defect of third order of the lens 5. l

'FIG 2 shows an electrostatic lens 14 of known construction Which is `combined with the netting lens 9 to `form @a single structural unit. The combined lenses shown'f'here can serve predominantly for the correction ofa beam of charge carriers of small aperture, since in general Ithe principal plane of the lens 14 will not coincide With-the plane 'of the netting 12. If ay beam of charge carriers of large aperture is to be corrected, it is necessary vto arrange the netting lens 9 in or in lthe immediate vicinity'of the principal plane of the lens 14. FIG. 3 shows an electromagnetic lens between the pole shoes 16 and 17 of which the netting'lens 9 is arranged. Insulator 18 supports the netting lens. The lens combination shown in FIG. 3 is particularly useful "for correcting a lbeam of charge carriers of large aperture since the principal plane of the lens coincides here with the plane of the netting 12.

FIG. 4 shows a netting lens 19 in which two ring electrodes 21 and 22 are larranged in front of the netting 20. Behind the netting 20, there is arranged another ring electrode 2'3. An insulator 24 serves to insulate the individual electrodes from each other.

' The netting lens shown here makes it possible better to control the charges induced in lthe netting than with a netting lens equipped with only two ring electrodes, such as shown for instance in FIGS. 1 to 3. In order to control t the space charge, the ring electrodes 21 and 22 are placed under different voltage.

If, for example, the electrode 22 is biased slightly positive with respect to the netting, the voltage of the electrode=21 is strongly negative with respect to the netting, and the electrode 23y is maintained at the netting potential, vna slightly positive charge will be induced in the center of the net-ting and a slightly negative charge in its outer zones. In between, there is a region which is practically free of charge. In this way, the correction is effected without the induced charges on the individual wires of theznettingtcausing any noticeable field disturbance in the vicinity of the individual netting wires. Thus a disturbingly' large lens eiect of the individual netting meshes is prevented. In the case'of the netting lenses shown in FIGS. 1 to 4, a foil with holes can be provided instead of the netting. Such a lens is shown in FIG. 5. The netting consists of a sheet y25 which is perforated by holes 26 of predetermined size. The lens itself is of the same co-nstruction as the netting lens 9 shown in FIG. 1. All the holes 26 in the'sheet may have the same diameter. It is, however, also possible to provide holes the diameters of which vary with its distance from the axis. For example the diameter of the holes 33 near the axis may be greater than the diameter of the holes in the outer peripheryof the sheet.

FIG. 6 shows another netting lens in which instead of the fnetting there is provided a sheet 27 which is of different thickness at different points.

In many cases, it may be advantageous instead of the `netting lens 9 shown in FIG. 1 which containsl a taut netting 12 to provide a netting lens of arched netting.

Such a lens is shown in FIG. 7 which clearly shows the curvature of the netting 28. l0f course, instead yof the netting 23 an arched sheets'with holes of denite size can also be provide d.

FIG. 8 shows another embodiment of a netting lens with only two ring electrodes. In the `case of this netting lens, it is seen to it by suitable dimensionings of the ring electrodes and by selection of the voltages applied to them that the total charge induced by the ring electrodes on the part of the netting traversed by the beam is small. The netting itself is designated'301while'the upper ring electrode bears the designation 31 and the lower ring electrode the designation 32. The electrode 31 is at negative potential and has a smaller opening serving for the passage of the beam than the lower electrode '32 which is at positive potential. The netting 30 is so arranged that its distance from the ring electrode 31 is greater than its instance from the ring lelectrode 32. In the center of the netting 30, the positive charge density induced by the upper ring electrode 31 predominates, while in the outer parts of the netting the negative charge induced by the lower electrode 32 predominates. Accordingly, in the center the netting produces a positive power of reraotion which decreases towards the outside and even becomes negative further towards the outside.y

I-n the annular regi-on in which the charge densities induced from both sides :are equal and opposite to each other, there is no lens effect of the individual netting meshes.

- By the apparatus described and shown, it is possible to produce a small charge-carrier beam spot of high intensity. In this way, itis possible to obtain a high surface intensity with small absolute ldiameters of the beam. The intensity over the cross-section of the beam should be so distributed that it drops rapidly at the edges. Such an intensity distribution is ofparticularly great importance when working material with beams of charge carriers, since here it is important to combine as much energy `as possible within the limits of the place being machined.

This invention may be variously modified and embodied Within the scope of the subsequent claims.

What is claimed is:

1. Apparatus for compensating for the apertural defeet of [an `electron optical lens of rotational symmetry which is free of space charge, comprising a corrective lens, said corrective lens being positioned near the principal plane of said electron optical lens, said corrective lens comprising a rst and second ring electrode positioned inparallel planes perpendicular to the axisl of the beam, a netting clamped between said ring electrodes and insulated therefrom, and means `for biasing each of said ring electrodes to induce a charge `distribution on said netting varylng with, the distance from the center of the netting, the total charge so induced on the netting traversed by the beam being so slight that no disturbing 'lens effect of the individual netting meshes occurs. Y

2. Apparatus according to claim l which includes a third ring `electrode positioned parallel to said iii-st ring electrode and insulated therefrom and which includes means for biasing said third electrode.

l 3. Apparatus according to claim 2, in which said first and thirdring electrodes [are positioned in front of the netting, las seen in the direction of the beam, and in which said electrodes are biased so that said third electrode is negative with respect to the netting, said first electrode 4is positive with respect to the netting and is of lower absolufte magnitude than said negative bias on said third electrode, and second ring electrode' is at the netting potential.

4. Appanatus according to claim l, in which the upper ring electrode as seen in the direction ofthe beam is biased at a negative potential relative to the netting and has a smaller `aperture for the passage of the beam than the lower ring electrode, the lower ring electrode is biased at 5 a positive potential with 'respect to fthe netting, and the 7. Apparatus according Ito claim 1, in which the netdstance Iof the netting ffrom the upper ring electrode is ting iS archedgrea-ter than `tits ldistance imm the lower ring electrode. 8- Apparatus aCOOTdIIg T0 Claim 5 Il which the ffoil .5. Apparatus according [to daim 1, in which Said ne@ varies in thickness along a madius rom the laxis. ting is formed from a thin erforated foil. 5

6. Apparatus according |50 claim 5, in which the 'diami References Cited m the me of thls patent eter of the holes contained in the `foil varies with the dis- UNITED STATES PATENTS 

1. APPARATUS FOR COMPENSATING FOR THE APERTURAL DEFECT OF AN ELECTRON OPTICAL LENS OF ROTATIONAL SYMMETRY WHICH IS FREE OF SPACE CHARGE, COMPRISING A CORRECTIVE LENS, SAID CORRECTIVE LENS BEING POSITIONED NEAR THE PRINCIPAL PLANE OF SAID ELECTRON OPTICAL LENS, SAID CORRECTIVE LENS COMPRISING A FIRST AND SECOND RING ELECTRODE POSITIONED IN PARALLEL PLANES PERPENDICULAR TO THE AXIS OF THE BEAM, A NETTING CLAMPED BETWEEN SAID RING ELECTRODES AND INSULATED THEREFROM, AND MEANS FOR BIASING EACH OF SAID RING ELECTRODES TO INDUCE A CHARGE DISTRIBUTION ON SAID NETTING VARYING WITH THE DISTANCE FROM THE CENTER OF THE NETTING, THE TOTAL CHARGE SO INDUCED ON THE NETTING TRAVERSED BY THE BEAM BEING SO SLIGHT THAT NO DISTURBING LENS EFFECT OF THE INDIVIDUAL NETTING MESHES OCCURS. 